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Discontinued operations (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Dec. 31, 2011
Discontinued Operations and Disposal Groups [Abstract] |
Arbitration charge related to a guarantee | $ 21.0
Arbitration charge after tax | $ 13.0

X
- Definition Arbitration charge after tax + References No definition available. + Details
Name: | mdu_ArbitrationChargeAfterTax
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Arbitration charge related to a guarantee + References No definition available. + Details
Name: | mdu_ArbitrationChargeRelatedToGuarantee
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_DiscontinuedOperationsAndDisposalGroupsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration